The present invention relates to methods and apparatus for measuring and monitoring the frequency of alternating current (AC). More specifically, the present invention relates to methods and apparatus for converting analog values of the alternating current to digital form and then electronically performing Fourier transform computations to provide frequency measurements, monitoring and protection in frequency-sensitive systems. The invention also relates to electrical system utilizing Fourier-transform-based monitoring and protection methods and apparatus.
Hitherto it has been proposed to sample the analog AC waveform and compute the Discrete Fourier Transform (DFT) on the waveform to obtain frequency information. However, sampling over many cycles of the waveform has been involved in the prior art in order to obtain reasonable accuracy of frequency measurement, thus limiting the swiftness with which frequency measurement can be accomplished by the Fourier transform technique. Discussions of the Discrete Fourier Transform have also recognized as disadvantage in that the calculation process itself introduces a phenomenon known as "leakage" which causes spurious indications of d.c. (zero frequency) and harmonic content in a waveform which is actually a single-frequency sinusoid only. Thus, the Discrete Fourier Transform has been recognized to have complications which make it difficult to use for frequency measurement.
On the other hand, the need for and commercial potential of inexpensive frequency-measuring apparatus is increasing with each passing year. For example, in the electrical power generation, transmission, and distribution fields a decrease in the frequency of the AC electric power waveform can indicate excessive loading of the generating equipment by increased energy consumption. In order to prevent catastrophic blackout occurrences, electric power systems must be provided with means of detecting such decreases in frequency and shedding load in an effective manner until the systems can adjust to the changed loading conditions. Frequency-measuring devices for monitoring and keeping records on the performance of power systems are also necessary for proper maintenance and planning of such systems.
The DFT can be electronically computed in a manner efficient of time by means of any of a class of algorithms known as the Fast Fourier Transform (FFT). With the advent of large-scale-integration (LSI) computer chips with substantial digital computing power in truly miniature dimensions at attractively low expense, the DFT could be calculated were it not for the recognized practical and inherent disadvantages that have hitherto deterred its use in the frequency measuring and monitoring field.